1. Field of the Invention
This inventions relates to automatic control over video cameras and, more particularly, to apparatus for controlling the lens and aperture of a video camera.
2. Description of the Prior Art
Video cameras are known with motor-driven lenses which effect automatic focus control as well as operator initiated zoom lens control. Such cameras also are provided with automatic aperture control arrangements. To provide greater flexibility and permit the use thereof under diverse conditions, such video cameras are operable with removable lenses. Consequently, different lenses having different focal lengths and different ranges of aperture openings (commonly referred to as f-stops) may be used with a given video camera. Depending upon the distance of the imaged object from that camera and the lighting conditions thereof, the user is provided with a wide selection of lenses for use.
Zoom lenses having selectively variable focal lengths also are available for use with video cameras to permit artistic effects by "zooming" in or out, and also to permit the use of one lens to image an object whose distance from the camera may change.
Video camera that are designed for fixed lenses as well as those that are designed for removable lenses advantageously are adapted to control the focusing and/or "zooming" effect by means of one or more drive motors. To facilitate the use of several different removable lenses, it has been proposed to incorporate the lens drive motor and the lens elements in a common housing which, together with control circuitry is referred to generally as a lens device. One example of a lens device having both a focusing motor and a zoom motor is described in Japanese published patent application No. 63-042274. The video camera with which this lens device is used includes circuitry to produce a focus control signal and a zoom control signal which are coupled to the focus and zoom motors in the lens device by an electrical connection which is formed when the lens device is mounted on the Camera. The focus motor thus adjusts the focus condition of the lens device and the zoom motor controls the focal length thereof and, thus, the imaging angle of view.
In a typical arrangement of the aforementioned combination of a video camera and replaceable lens device, the control signals supplied from the camera housing to the lens device generally are simple "stop/go" signals. For example, the focus control signal may exhibit a constant magnitude which is used to drive the focus motor and a polarity which determines the direction in which that motor is driven. Likewise, the zoom control signal may exhibit a substantially constant magnitude to drive the zoom motor and a polarity which determines whether the motor is driven to "zoom in" or "zoom out".
Since the lens control signal (i.e. the focus control signal or the zoom control signal) is of a constant magnitude, such arrangements assume that the lens drive motors will be driven at a substantially constant speed, whereby uniform focus and zoom adjustments are effected Uniform lens adjustment speeds for different replaceable lenses and for different operating conditions are desired. The user may be accustomed to a particular focus adjustment speed or zoom adjustment speed; and, therefore, unanticipated speed variations are not welcome. However, the assurance of uniform adjustment speeds generally requires the use of a reference and feedback control to make certain that the adjustment speed conforms to the reference.
In an attempt to miniaturize the motor control circuits incorporated into replaceable lens devices, reference generators normally included in the lens device for motor speed control have been omitted. Even without a reference generator, it had been assumed that by using control signals of constant magnitude, uniform speeds nevertheless may be attained and feedback control was not needed. But, because of changes in ambient temperature, changes in the load presented to the lens drive motors and variations in the operating characteristics from one replaceable lens device to another, the magnitude of the lens control signal may not be constant but, rather may vary. Furthermore, even if the magnitude of the lens control signal remains constant such that the signal is sufficient to control the focusing and zooming of one lens device at a proper rate of speed, the control signal may be inadequate for accurate control at desired speed of another lens device.
Consequently, focus and zoom adjustment speeds may vary from one lens device to another and, as the environmental conditions vary, the focus and zoom adjustment speeds for a given lens device may fluctuate. Hence, stable adjustment speeds of such lens devices may not be achieved.
A similar disadvantage is present in the aperture control arrangement for such replaceable lenses. That is, the aperture opening of the iris normally provided in the lens device may be increased or decreased at a rate of speed that is not stably controlled.
Moreover, for aperture control, it often is desired to reduce the adjustment speed of the aperture when the opening thereof approaches a desired value, or f-stop. That is, overshoot, which may occur if the aperture is adjusted at a constant speed, is prevented, or at least minimized, if the adjustment speed is reduced as the aperture approaches a desired opening. For example, if the aperture opening is too small for the brightness level of an imaged object, it is desirable to increase the aperture opening at a relatively faster rate initially and then reduce the rate at which the aperture is opened when the proper f-stop is approached, thereby preventing or minimizing any overshoot in such aperture adjustment.